Rocker Arm Ratio Explained
Choosing the correct rocker arm ratio for an engine is a vital part of any engine build or upgrade. There are different ratios available and the purpose of this Jegs tech tip is to help you make the right choice for your engine.
One of the most popular ratios is 1.5, and what exactly does that mean? It means the rocker arm will open the valve 1 and a ½ times the amount the cam moves the lifter and the push rod, or what cam manufacturers refer to as cam or lobe lift. So if we have a camshaft with a cam/lobe lift of .300 and choose a ratio of 1.5 it will open the valve .450 or .300 x 1.5 = .450.
So if a manufacturers rocker arm ratio is 1.5 and we decide to replace them with a ratio of 1.6 what exactly does that do? By doing the math we see the valve now opens .480 so we have increased the valve lift of our camshaft without affecting camshaft duration. (The valve travels a longer distance in the same amount of time).
If you only know your camshafts valve lift and not its cam/lobe lift you will need to divide the valve lift by the original ratio to get the cam/lobe lift and then multiply your new ratio to get your new valve lift.
An example would be .450 ÷ 1.5 = .300 then .300 x 1.6 = .480 is my new valve lift.
Typically increasing rocker arm ratio will increase horsepower at higher RPMs but there are some drawbacks such as less power at lower RPM’s and more stress on the engines valve train as well as the rocker arm itself, and as always make sure there is sufficient valve to piston clearance.
On a rocker arm with a ratio of 1.5
The valve side Y will be 1.5 times longer than the pushrod side X